Technology for mounting with higher densities on part substrates has been progressing, with regard to electronic devices such as cell-phones, personal computers and digital cameras, along with the development of miniaturization technology with respect to surface mounting technology in recent years. In this scenario, with regard to capacitor elements of electronic parts, intensive investigation is being carried out in particular into aluminum, tantalum, and niobium electrolytic capacitors for which miniaturization and capacity enlargement are possible.
Electrolytic capacitors are configured from a large surface area nanoporous anode, an oxidized film dielectric on the anode, and a cathodic conductive layer of a conductive polymer on the dielectric. Since the capacity of electrolytic capacitors increases in proportion to dielectric area, with tantalum, for example, nanoporous tantalum or the like, having an average pore diameter of 100 nm to 500 nm, being produced by sintering nanoparticles, is used in recent years.
In conventional processes for forming electrolytic capacitor elements, a nanoporous anode is electrolytically oxidized in an aqueous electrolyte to form an oxidized film dielectric (20 nm to 200 nm). Then a manganese dioxide film, which is a preliminary conductive layer, or a conductive polymer film (100 nm to 1000 nm) is formed on the oxidized film by a chemical polymerization process. A conductive polymer film is electrolytically polymerized (1 μm to 10 μm) using the preliminary conductive layer as an anode, which is cumbersome.
In view of future increases in capacity, it is considered that nanoporous anodes having an average pore diameter of no more than 100 nm will be used; however, in cases where anode substrates having such a small pore diameter are used, it is difficult to form the conductive polymer of a cathode layer as far as the inside of the pores, with a uniform thickness, by a process of forming a preliminary conductive layer, using conventional chemical polymerization processes; thus there is a risk of a decrease in the rate of capacity development an increase in equivalent series resistance (ESR).
Therefore, as a process to form directly the conductive polymer film on the anode substrate without using the preliminary conductive layer, the present inventors have proposed a process such that an oxidized film and polypyrrole are simultaneously formed on a nanoporous tantalum anode by electrolysis using an aqueous solution containing a surfactant salt as an electrolyte (e.g. see Non-Patent Document 1, Patent Documents 1 and 2). In accordance with the process, although the polypyrrole can be formed as far as the inside of the fine pores, the polypyrrole inside the pores represents a nonuniform island-like condition. It is therefore believed that it is necessary to polymerize uniformly as far as the inside of the pores.
On the other hand, electrolysis solutions, which dissolve electrolyte salts in organic solvents, represent a problem in that the firing point is lower due to volatile organic solvents, or a problem in that long-term reliability is insufficient due to the possibility of generating liquid leakage; therefore, electrochemical devices have been investigated that use an ionic liquid, that is liquid-like at room temperature, for an ion conductor, as a material to improve these problems. In particular, an electrochemical device is proposed that exhibits excellent charge-discharge cycle behavior and a longer operating life by use of an electrolytic polymerization product of polymerizable compounds (monomers) such as pyrrole and thiophene in an ionic liquid as a conductive polymer (e.g. see Patent Document 3). However, application of conductive polymer films, prepared by use of ionic liquid electrolytes, for electrolytic capacitors has not been tried yet.    Non-Patent Document 1: 71st Meeting of Electrochemical Society, Proceedings p. 279 (2004)    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2001-223139    Patent Document 2: Japanese Unexamined Patent Application Publication No. 2001-223140    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2003-243028